In financial transactions whereby securities are purchased by the Board under an agreement providing for the resale of the securities to the original seller at a stated price, the Board shall take physical possession of the securities, directly, by its custodian bank or through a neutral third party: Provided, That an agreement with a neutral third party may not waive liability for the handling of the securities: Provided, however, That when the board is unable to take possession, directly, by its custodian bank or through a mutual third party, the Board may leave securities in a segregated account with the original seller, provided the amount of the securities with any one seller may not exceed $150 million.